This invention relates to the forming of readable (not necessarily optically) precision marks, a pattern or other indicia on metrological and like equipment by means of irradiation of laser light. In particular but not exclusively it relates to the formation of a pattern of marks on an object e.g. graduation marks on a scale to be used for metrological purposes.
Some major problems encountered when using a beam of laser light for marking a surface or subsurface of a material are: the dimensional control of the laser light with respect to that surface/subsurface e.g. to maintain accurate spacing of the marks; the correct selection of parameters of the light e.g. beam fluence (energy per unit area) duration of exposure of the surface/subsurface and; the adaptation of the marking process to suit different applications such as the ability to mark flat surfaces as well as curved surfaces; and the ability to produce patterns of marking having different pitch lengths. As well as these problems heat build-up at the laser exposed area is problematic because this changes the dimensions of the resultant scale and leads to inaccuracy. Particularly problematic is the heat build-up in thin metallic metrological scales, like stainless steel which can buckle and become brittle as a result of excessive heat.
The production of measurement scale using a laser light to mark its surface has been considered previously. In U.S. Pat. No. 4,932,131 an in-situ scale writing or calibration technique is used. A reference is used to lay-down marks or correct any deficiencies in the scale. A laser is used to read and write a scale but there is no disclosure of the method for doing this, and no mention of overcoming thermal problems.
JP 5169286 shows a method of obtaining a marking perpendicular to the direction of travel of a measurement scale which is being marked using a laser. In JP 5169286 there is no discussion of thermal problems and no mention of the apparatus which controls the firing of the laser or the beam's position relative to the scale.
It is known that there are effectively two different mechanisms for pulsed laser ablation of materials and that the key factor for determining which mechanism is employed is the pulse length. Essentially, for pulse lengths above approximately 4 picoseconds, the material is melted and then boiled off from the surface with considerable transfer of thermal energy into the remaining material. For pulse lengths below approximately 4 picoseconds (i.e. ultrashort pulses), the molten stage is omitted with the material either (depending on correct understanding of the mechanism) being sublimated straight from the solid to the gaseous or ejected from the substrate as minute solid particles. When ultrashort pulses are used, the amount of thermal energy transferred to the material is significantly reduced.